The present invention relates to an image forming apparatus conducting image forming through an electrophotographic system, and in particular, to an image forming apparatus wherein double-sided image forming is conducted by forming toner images on both sides of a transfer material and by heat-fixing them collectively.
Heretofore, for double-sided copying to form toner images on both sides of a transfer material, there has been employed a method wherein an image on one side which is formed on an image forming body is transferred and fixed on a transfer material, then the transfer material is stored temporarily in a double-side reversing and sheet-feeding device, and then the transfer material is fed out of the double-side reversing and sheet-feeding device in synchronization with an image formed on the image forming body successively so that the image on the other side is transferred and fixed on the transfer material.
In the double-sided copying apparatus of this kind, a transfer material needs to be fed to a double-side reversing and sheet-feeding device and needs to be conveyed to pass through a fixing device twice, as stated above, resulting in a longer conveyance distance for the transfer material and resulting in longer processing time for copying. Further, reliability in conveyance of a transfer material is low, causing a jam, because the transfer material which tends to be curled after passing through the fixing device is fed again.
In opposition to the foregoing, TOKKOSHO Nos. 49-37538, 54-28740, TOKKAIHEI Nos. 1-44457 and 4-214576 disclose those wherein toner images are formed on both sides of a transfer material to be fixed collectively.
However, it is not easy to conduct fixing collectively for the transfer material having on its both sides toner images. To conduct fixing collectively for the transfer material having on its both sides toner images, it is necessary to provide heating means on both sides of the transfer material, which inevitably results in big power consumption in a fixing means because power is supplied to both heating means. Further, in this method, an interval for conveying a transfer material to a fixing device in the case of single-sided copying is different from that in the case of double-sided copying, and therefore, fixing failure or waste of electric power is caused in the conventional fixing device.
The first object of the invention is to solve the above-mentioned problems and to provide an image forming apparatus wherein excellent fixing can be conducted with less power consumption even when a transfer material having toner images on its both sides is collectively fixed and when a transfer material having a toner image on its one side is fixed, without reducing the number of transfer materials to be processed.
Inventors of the invention have studied color image forming on both sides of a transfer material and on one side only of a transfer material, by making an image forming apparatus wherein a plurality of toner image forming means each being composed of a charging means, an imagewise exposure means and a developing means are arranged around the first image carrying means, the second image carrying means is provided against the first image carrying means, then superposed color toner images representing a reverse side image are formed on the first image carrying means and these color toner images are collectively transferred onto the second image carrying means temporarily, then superposed color toner images representing an obverse side image are formed on the first image carrying means, and after transferring the toner images on the first image carrying means onto the obverse side of the transfer material and transferring the toner images on the second image carrying means onto the reverse side of the transfer material, toner images on the transfer material are fixed to form a double-sided color image.
In the aforesaid image forming apparatus and the image forming method, image forming timing and transfer material supplying timing are controlled so that toner images representing a reverse side image held on the second image carrying means and toner images representing an obverse side image held on the first image carrying means may be synchronized with a transfer material in the transfer area, for the purpose of aligning the leading edge of the obverse side image and that of the reverse side image on the transfer material. Namely, when forming images on both sides of a transfer material, a time interval from the start of forming a reverse side toner image to the start of forming an obverse side toner image is made to agree with the rotation cycle of the second image carrying means regardless of a size of an image and of a length of a transfer material in the conveyance direction therefor, and supply timing for the transfer material is controlled in synchronization with timing for forming the obverse side toner image. For the reason of easy control of timing, even in the case of continuous image forming on only one side of a transfer material, a time interval from the start of toner image formation or the start for supplying a transfer material to the succeeding start of toner image formation or the succeeding start for supplying a transfer material is made to agree with the rotation cycle of the second image carrying means. Further, when forming images continuously on both sides of a transfer material, a time interval from the start of forming an obverse side toner image to the start of forming a reverse side toner image is also made to agree with the rotation cycle of the second image carrying means. Furthermore, even in the case of forming images continuously on both sides of a transfer material while making the second image carrying means to carry thereon toner images equivalent to plural sheets of the transfer material, an interval of forming toner images is determined based on the rotation cycle of the second image carrying means so that intervals for forming reverse side toner images or obverse side toner images which are formed continuously may be equal.
However, in the case of forming images continuously only on one side of a transfer material, if a time interval is made to agree with the rotation cycle of the second image carrying means when the restriction of control of timing for an image forming process or for supplying a transfer material is only way for the time interval for toner image forming or for transfer material supply, the number of sheets for image formation per unit time is reduced. When forming images continuously on both sides of a transfer material, if the time interval from the start of forming an obverse side toner image to the start of forming a succeeding reverse side toner image is made to agree with the rotation cycle of the second image carrying means, the number of sheets for image formation per unit time is also reduced in this case. Furthermore, even in the case of forming images continuously on both sides of a transfer material while making the second image carrying means to carry thereon toner images equivalent to plural sheets of the transfer material, if the interval of forming toner images is determined to be the same as the rotation cycle of the second image carrying means when the restriction of timing control for the image forming process or for supply of transfer materials is the only way for the forming interval for reverse side toner images or obverse side toner images to be formed continuously, the number of sheets for image forming per unit time is also reduced in this case.
The invention has been achieved in view of the technical problems stated above, and its second object is to provide image forming apparatuses in the following items 2-1-2-3.
2-1
An image forming apparatus wherein it is possible to improve the image forming speed in the case of forming images continuously on one side of a transfer material.
2-2
An image forming apparatus wherein it is possible to improve the image forming speed in the case of forming images continuously on both sides of a transfer material.
2-3
An image forming apparatus wherein it is possible to improve the image forming speed in the case of forming images continuously on both sides of a transfer material, while making the second image carrying means to carry toner images equivalent to plural sheets of transfer material.